The present invention relates to a method of effecting a split rolling of steel sheet suitable for slitting a wide steel sheet and rolling the split steel sheets.
Generally, the process for producing a steel sheet has the steps of forming blank slabs from a molten steel by means of a continuous slab casting apparatus, rolling the slab by a hot strip mill down to a thickness of 2 to 6 mm, pickling the rolled steel sheet, further rolling the steel sheet by a cold strip mill down to a thickness of about 0.1 to 2.0 mm and then effecting a surface treatment such as zinc plating on the steel sheet thereby to obtain the desired steel sheet product.
The series of equipment for conducting the above-mentioned steps usually have a capacity large enough to treat steel sheet of a large breadth ranging between 600 and 2000 mm. Actually, however, the equipment is operated to treat comparatively narrow strips of 900 mm wide or so which has the greatest demand, because the blank slab is formed at such a small breadth meeting the demand. This means that the capacity of the equipment, which is large enough to treat steel sheets up to 2000 mm wide, could not be fully utilized.
When the comparatively narrow strips are treated by the equipment which can accommodate strips of large breadth, the guide rolls and rolling rolls of the continuous casting apparatus are locally worn only at the breadthwise portions thereof contacted by the narrow material which is rolled into the slab. Therefore, the user is obliged to renew these rolls frequently, even though the other portions of the rolls are not worn. In consequence, the running and maintenance costs incurred when the narrow strips are rolled are several times as high as those incurred when the wide strips fully occupying the roll breadth are produced.
In order to improve the production efficiency of the steel sheet while reducing the installation cost by overcoming these problems, it is advisable to operate the equipment with strips having a large breadth and, in order to meet the demand, to split the same into the demanded sheet breadth at a step immediately before turned into final product, e.g. after the rolling by cold strip mill in the production of zinc-plated steel sheet.
Rolled steel sheet usually has a tapered edge referred to as "edge drop" along both side edges thereof. Namely, the rolling rolls, which have circular cross-sections are deformed under the reduction force to generate the edge drops at the boundary of contact between the roll and the steel sheet, i.e. at each side edge of the steel sheet. The edge drop is attributable also to a lateral or breadthwise plastic flow of the material during the rolling. The breadthwise plastic flow is heavy particularly in hot rolling. In some cases, the steel sheet having edge drops at both side edges is slitted by a known device such as a slit. In such a case, the slitted ends, i.e. the side edges or corners, of the slit sheets are naturally keen and the slit surfaces form a right angle to the plane of the slit sheets. When such a slit sheet having keen side edges is subjected to a plating with zinc, tin or the like, the flow of the plating solution is unsmooth at the keen edges of the sheet because the corners tend to be cooled rapidly. In consequence, the plating layer is expanded in the thicknesswise direction of the sheet along the side edges or corners to form an excessive layer generally referred to as "edge overcoat". The generation of the edge overcoat not only consumes the plating solution wastefully but the commercial value of the steel sheet product is deteriorated because the steel sheet coil undesirably has a frusto-conical shape due to the inflation at the portions where the edge overcoat is formed.